Seasonal affective disorder, or SAD, is a condition that causes lengthy bouts of severe depression during certain seasons of the year, especially winter.

What is going on in the body?

When a person has seasonal affective disorder, he or she experiences depression during a specific season. This condition may happen during any season, but it appears to be more common during the winter. Often SAD begins in the fall and continues through the winter until early summer.

What are the causes and risks of the condition?

Although the cause of SAD is not completely clear, one of the suspected causes is an increased level of melatonin in the blood system. Melatonin is a hormone secreted by a gland in the brain. Melatonin increases the need and desire to sleep. When it is dark, the body produces more melatonin. The more darkness in the day, the more melatonin is produced.

Less sunlight may also have an effect on a person's "biological clock." A person's body may not adjust to the change in routine due to time changes and daylight changes. Other causes of SAD may include changes in the body's temperature cycle and hormonal changes.

SAD is four times more common in women than in men. SAD sometimes occurs in children but appears to be more common after the age of 20 years. SAD also appears to be more common in people living in the northern states as compared to those in the southern states.